Crystallization apparatus



May 2, 1933. w. E. BURKE 1,906,534

CRYSTALLIZATION APPARATUS original Filed May 23, 192'! 2 sheets-sheet 1COOL 7? Coon/v6 M50/un coMPREasfb 4f/f? Cf/VTR/Fl/GE May 2, 1933.

W. E. BURKE CRYSTALLI ZATION APPARATUS 2 Sheets-Sheet 2 u 7f77/Q 7 If2Q; El f! 2g,

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Patented May 2, 1933 UNITED STATES PATENT OFFICE WILLIAM E. BURKE, OFTBONA, CALIFORNIA, ASSIGNOB TO AMERICAN POTASH` AND CHEMICALCORPORATION, OF NEW YORK, NVY., A CORPORATION F DELAWARE CRYSTALLIZATIONAPPARATUS Original application led Hay 23, Serial No. 193,673, PatentNo. 1,787,358, dated December 30, 1980.. Divided and this applicationled October 24, 1930. Serial No. 491,058.

This invention relates to improvements in apparatus for crystallizingdissolved crystallizable constituents from kliquors in which they aredissolved, for example, for crystallizing inorganic salts from saltsolutions, and is a division of my application Serial No. 193,673, filedMay 23, 1927, now Patent No. 1,787,356 issued Dec. 30, 1930. Theapparatus of the present invention, while being adapted for use in therecovery of crystallizable constituents generally, is particularlyadapted for use in connection with methods of crystallizing borax fromsolutions saturated or supersaturated with potassium chloride and boraXsuch as are described in my aforesaid application, which solutions areproduced when Searles Lake brine is concentrated, and the presentinvention will be described more particularly in connection with anapparatus for carrying out such methods.

One method commonly employed for crystallizing salts from solutionsthereof is to cool the solution, after bringing it tosaturation or nearsaturation by evaporation of the solvent, and to precipitate the saltcrystals because of reduced solubility at lower temperature. Ordinarilysuch operations are carried out in apparatus called crystallizers inwhich both the cooling and the orystallizing are efected, and theoperation is usually carried out on a batch basis, a batch of hotsolution being supplied to the crystallizer and cooled therein, thecrystallized product and remaining solution removed and another batch ofhot liquor supplied. Where the solution is cooled in the crystallizer,the cooling surfaces are rapidly covered with salt deposits which impedeheat transfer and which soon render the cooling ineffective unless theyare removed. Agitation is sometimes employed to retard this effect, butthere are certain dificulties incident to the means of agitation whichhave previously been proposed for this purpose. lVhere the agitation iseffected by the introduction of compressed air, the air tends to fornichannels through the crystal sludge as crystallization takes place sothat the agitation is neither uniform nor adequate, and is at the sametime difficult to control. Where mechanical agitation is employed, oneserious diilicultyis the tendency to freezing up of the agitator due toclogging of the agitator by the crystallized salt, particularly onstarting up after a shut down. Agitation by the introduction of liquidis difficult to. control and is not adapted for the production of auniform product as ordinarily applied. Commercially, it is frequentlynecessary that the product be in crystals of uniform size. Also, formany commercial purposes, large crystals are desired, and agitation asusuallI applied tends to produce small crystals. itherto large crystalshave b een obtained by slow Icooling of the solution, but for thetreatment of large volumes of -solutions this requires eithercrystallizers vvhich are too large or too many small crystallizers. Withbatch operation, moreover, there. 1s a loss of cooling and crystallizingtime in charging and discharging, and cleaning, the crystallizers.Normally, when a solution `saturated with more than one salt is cooled,the salts are crystallized and precipitated together. Certain salts,hdwever, tend to stay in supersaturated solution and in such cases aseparation or a partial separation of one or more of the salts can beeffected before the crystallization of the other or others begin.However, the solution remains saturated with the separated salt and infurther treatment to recover the salt remaining in the supersaturatedsolution this salt is usually obtained contaminated with the salt firstremoved. This entails further washing and refining operations, as wellas incidental operations such asI the rerunning of washing liquors. Thebrines obtained on concentration of Searles Lake brine are of complexcharacter, and the separation of their various constituents,particularly borax and potassium chloride, has been further complicatedby` factors such as those outlined above. Hitherto, its hasbeenjnecessary in ordinary operachloride and is frequently in the formof small and nonuniform crystals.

According to the present invention, the crystallization is carried outby cooling the solution to a temperature below saturation so that it issupersaturated with the salt, or other material, it is desired tocrystallize and then flowing the supersaturated solution through asludge or suspension of crystals of. the salt or material whilemaintaining controlled agitation both mechanically andby liquorintroduction. The operationis advantageously carried out in a continuousmanner supplying supersaturated solution and withdrawing decrystallizedsolutionand the crystal sludge or suspension in a progressive way. Thecooling operation is carried out separately from the crystallizingoperation. Difficulties due to crystal deposits on cooling surfaces inthe crystallizer are thus avoided and the operation of the crystallizeris improved. The cooling operation also is carried out so that a minimumof crystallization is effected therein so that difficulties tosalting-up in the cooler are also reduced. The cooling is advantageouslycarried out as rapidly as possible to prevent or inhibit the formationof fine crystals or any large amount of crystals in the coolingoperation, and in the cooling operation the temperature isadvantageously reduced as far as possible below saturation, withoutundue crystallization, so that the cooled liquor supplied to thecrystallizer is supersaturated to a maximum extent. iVhen thesupersaturated solution is brought into contact with the crystal sludge.crystallization takes place largely as a crystal growth on the crystalsin the sludge so that large crystals can be obtained. The large exposedsurface of the crystals in the sludge also promotes rapidcrystallization and enables the crystallization to be carried out at anin` creased rate. The crystal sludge is advantageously maintained in avertically arranged body to the lower end of which supersaturated-solution is supp-lied. decrystallized solution being withdrawn from theupper end, and from the lower end of which the accumulating crystalsludge is removed either continuously or intermittently. In carrying outa crystallization process with the present apparatus, the crystal sludgeor suspension is agitated both mechanically and by the introduction ofthe supersaturated liquor in an improved manner servingA to maintain thecrystals in suspension. l/Vith the sludge or suspension in a verticalbody, the crystals are progressively graded in size through the heightof the body. smaller crystals being suspended nearer the top and largercrystals nearer the bottom, and this graduation of crystal size can becontrolled bymeans of the agitation. The suspension and grading of thecrystals can be effected under certain conditions by agitation due toliquor introduction and flow through the body alone, but

usually the rates of flow required are so highy as to require anexcessive height oi' sludge the operation in a body relatively high or pdeep as compared to its cross-section. The larger crystals thus settleto the lower end of the body whence they can be withdrawn, and it willalso be apparent that the apparatus of the invention enables the processto be carried out with the production of crystals very uniform as tosize. The smaller crystals form or separate in the upper end of the bodywhere they also tend to grow and to settle through the body replacingthe seed crystals iu the sludge as the crystal productI is withdrawn.With rapid cooling of the liquor supplied to the crystallizingoperation, the. formation of small crystals is avoided or reduced andany tendency lfor the decrystallized liquor to carry oitl `very finecrystals in suspension is correspondingly reduced, but it isadvantageous to carry out a settling op eration in connection with thecrystallizing operation and to return to the crystallizing operationvery line crystals removed from the end liquor therein. For example,less vigorous agitation may be employed at the upper end of the liquidbody in the crystallizer or the upper end of the body may be of enlargedcross section providing for separation and direct return to thecrystallizing operation of very fine crystals. By effecting thecrystallization from supersaturated solution upon seed crystals in anapparatus made in accordance with this invention an improved separationbetween two or more crystallizable constituents can be obtained as thecrystals grown on the seed Crystals tend to be substantially of thecomposition of the seed crystals, and the invention is thus useful ineffecting such separation even where the solution is supersaturated withmore than one crystallizable constituent. The crystallizing operationcan be further controlled byregulated dilution of the liquor, eitherbefore it is cooled or as it is supplied to the crystallizer, withsolvent or with otherappropriate solutions. For example, solvent alonemay be supplied to reduce the degree of supersaturation particularlywhere the solution contains two or more crystallizable constituents; or,where the solution is saturated or supersaturated with one salt andsupersaturated with another, it may be diluted with a solution whichdoes not contain or which is not saturated with the first salt and whichis saturated or supersaturated with the second to promote separation ofthe second salt.

The apparatus of the present invention comprises a cooler and a separatecrystallizer.l yThe cooler may be of any ofthe usual types which areadapted to cool the liquor rapidly, for example double tube coolers maybe employed, and forvcontinuous operation it is advantageous to providetwo or more coolers in conjun'tion with each crystallizer so that theoperation can be carried on while one or more of'the coolers-is beingcleaned. The invention includes an improved crystallizer of specialvalue for carrying out the process described above.

The crystallizer of the invention comprises a drum or cylinder with anapproximately vertical axis provided with a mechanical agitator. Thecylinder is :ulvantageously relatively long with reference to its diame-F ter. The agitator may consist of a series of arms arranged atintervals on a shaft eX- tending through the cylinder and adapted toagitate liquor in the cylinder without iving it any substantialcomponent of vertlcal motion. Scrapers may also be provided as part ofthe agitator to remove from the walls of the cylinder any saltscrystallizing thereon. An outlet is arranged for removing crystal sludgefrom the lower end of the crystallizer and the agitators may also beprovided with means for stirring up the sludge, settling in the lowerend of the crystallizer to promote its ready discharge and to inhibitany tendency toward freezing up of the agitator. One or more liquorinlets may be a rranged either immediately above the crystal sludgeoutlet or at some intermediate point along the cylinder. One factor indetermining the number of liquor inlets is the volume of liquor to besupplied as related to the tendency of constituents dissolved in theliquor to crystallize from it under the conditions of operationmaintained. The number and area of liquor inlets are advantageouslyproportioned so that the rate of flow of liquor through them retards orprevents-any clogging at this point due to crystal formation.

The agitation of the contents of the cylinder-V effected by themechanical agitator is further assisted by the fiow of introduced liquorupwardly therethrough, and this effect may be increased by dischargingthe liquor into the cylinder tangentially. Another way to introduce theliquor is through the agitator shaft and openings arranged thereon oropenings arranged on arms on the agitator shaft and rotated therewith.For the separation of line crystals from the end liquor and for theirreturn to the crystallizer proper, a. settler is advantageously arrangedat the upper end of the crystallizing cylinder. This settler may be ofincreased area to retard the rate of liquor fiow therethrough and thesettler construction is advantageously such that its sides slope steeplyto the upper end of the cylinder. It is also advantageous` to providemeans for propelling liquor and any settled crystals adjacent the sidesof the settler toward the crystallizing cylinder, and such means may becarried by and operated with the agitator shaft. A trough or launder maybe provided for the removal of liquor from the end of the settler. Theinvention includes the improved crystallizer as well as the combinationof the crystallizer with a separate cooler. The apparatus of thisinvention has several advantages, for example, it is compact and adaptedto continuous operation, and has a high capacity.

The invention will be further described in connection with theaccompanying drawings which illustrate in a diagrammatic andconventional manner apparatus embodying the invention. It is intendedand will be understood, however, that this illustration and the furthermore detailed description are for the purpose of exemplification andthat the invention is not limited thereto.

In the accompanying drawings:

Fig. 1 diagrammatically represents in elevaton a cooler and crystallizerembodying the invention, and somewhat in the way of a flow sheet, oneway of carrying out the process of the invention, and

Fig. 2 represents in vertical section and in somewhat more detail animproved crystallizer embodying the invention and adapted for use inpracticing the invention.

lVith reference to Fig. `1, the apparatus there illustrated comprises acooler 1, a crystallizer' 2, a settler 3, and a centrifuge 4. Inoperation, liquor is supplied to the cooler through connection 5 and isdischarged from the cooler and introduced int-o the lower end of thecrystallizer through connection 6. The cooler illustrated is of thedouble tube type, the liquor being circulated through the inner tube 7and a suitable cooling medium, such as refrigerated brine, beingcirculated through the outer tubes 8. A sludge of crystals of the.constituents of the liquor to be crystallized is maintained in thecrystallizer, and the cooler liquor, supersaturated with thisconstituent., introduced into the lower end of the crystallizer throughconnection 6. is flowed upwardly through this sludge, and the end liquoris removed from the settler 9 arranged on the upper end of thecrystallizer through the launder 10 and the connection 11. The crystalsludge in the crystallizer is agitated both by a mechanical agitator 12and by the upward How of liquor through the crystallizer. The crystals'are thus maintained more or less in suspension in the liquor in thecrystallizer, but, as the crystals grow in contact with thesupersaturated liquor, the larger crystals settle towards the bottom ofthe crystallizer whence they are removed as they accumulate throughconnection 13. The connection 13 discharges into a cone set-tier 3 inwhich a rough separation of liquid from the vcrystalled material iseffected, a compressed air inlet 14 being arranged for agitatin" thecontents of the cone settler and a llquid draw-off connection beingprovided at 15.

From the settler, the crystallized material is discharged throughconnection 16 into the centrifuge 4 for drying the crystalline product.The cooler, the conesettler, and the centrifuge may be of any usualsuitable con-` struction and operation. The eryst-allizer, and combinedsettlei for the separation of line crystals from the end liquor, isillustrated more in detail in Fig. 2.

Referring to Fig. 2, the crystallizer comprises a vertically arrangedcylindrical shell 17 through which an axial shaft 18 extends. This shaftis supported .at its lower end on a thrust bearinof which is protectedwith suitable stuffing boxes as shown at 19. `This shaft is driven fromits upper end by suitable gearing as shown at 20 and an intermediateguide bearing is provided at 2l. Arranged on the shaft 18 at severalpoints throughout the length of the shell 17 are agitating arms 22.Scrapers 23, carried by the ends of the ,arms 22, are also provided forremoving any material crystallizing on the interior of the shaft 17 Asettling chainber 24 of generally conical section is arranged at theupper end of the shell 17, and a launder 10 is arranged about the upperend of this settling chamber for the separation and removal of endliquor, the launder discharging through pipe 11. A pair of agitatorwheels 25 are arranged on the shaft 18 within the settling chamber andvanes 26 are provided on these wheels arranged between the outside ofthe wheels and the walls of the settling chamber. These van-es areinclined to give a downward componentl of motion to the liquor, and anysuspended or precipitated material, between the wheels-and the walls ofthe settling chamber when the agitator is in operation. The lower end ofthe shell 17 is closed by a cone shape-d member 27 in which the largercrystals tend'f to accumulate when the crystallizer is in operation. Astirrer comprising a number of rods 28 is arranged on and operated bythe shaft 18 within the chamber formed by this conical m-ember formaintaining the crystalline sludge in condition such that it can bedischarged continuously or intermitt-cntl as desired, through the outlet13. The liquor inlet connection 6 may also be arranged to discharge intothis chamber as shown, or it may be arranged to discharge at someintermediate point along the shell 17.

In the treatment of Searles Lake brine, the raw brine may beconcentrated approximately to saturation with potassium chloride byevaporation at a high final tempera- -ture as has previously beendescribed. By

rapid cooling of the brine, for `example, from about 110o C. to about 30C., in less than 3 hours, after bringing the brine to saturation withpotassium chloride at the higher temperature, the potassium chloridepresent in excess of that soluble at the lower temperature may becrystallized and separated from the brine before the borax begins tocrystallize. One particularly advantageous method and apparatus forcarrying s and,in the apparatus illustrated, is intro,

duced into the lower end of the crystallizer through connection 6. Asludge or suspension of borax crystals is maintained in theerystallizer. Initially, this sludge is supplied to the crystallizerfrom some outside source, but in continued operation the sludge isreplaced from the fresh brine as part of the operation as will appear.The cooled brine flows upwardly through this crystal sludge, the sludgebeing maintained in agitation both by the flow of brine through theerystallizer and by the mechanical agitator 12, which for example may bedriven at a rate in the neighborhood of one revolution per minute.A InContact with the sludge of borax crystals in the lcrystallizer, boraX iscrystallized from thecooled brine supersaturated with borax, thecrystallization taking place largely as a growth upon the crysgrowingcrystals seek a level which corre` sponds approximately to their size sothat 'the crystallizer' acts to grade the crystals as to size yieldingborax crystals of uniform size. As the brine flows upwardly through thecrystallizer, its content of borax in excess of saturation graduallycrystallizes so that the finer crystals in the upper end ofthe'crystallizer tend 'to increase in size as well as the coarsercrystals in theI lower end of the crystallizer where the brine is lirstintroduced. These growing iner crystals also settle gradually toward thelower end of the crystallizer so that a continuous supply of seedcrystals of borax is maintained. Very fine crystals of borax are finallyprecipitated from the brine in the settler 24 whence they are returnedto the upper end of the crystallizer where they also tend to grow and tosettle through the crystallizer. The brine supersaturated with boi-ax isthus flowed upwardly through and in counter current to a downwardlymoving sludge or suspension of borax crystals. The end liqthe brine mayalso be carried out in two or more steps instead of in a singleoperation.

In such progressive crystallization, the brine discharged from thecrystallizer through connection 11 may be further cooled and againflowed through a ksludge of borax crystals. The second crystallizationmay be carried out in another crystallizing unit or the brine, or partof it, after further cooling may. be recirculated through the samecrystallizer. In a continuous Operation, a part of the brine dischargedfrom the crystallizer may be so further cooled and recirculated throughthe crystallizer in a continuous manner. In one method of operationdescribed in application Serial No. 690,946, the brine from theevaporators remaining after separation of potassium chloride and boraxis recirculated through the evaporators together with fresh raw brine,and the present invention may be employed with advantage in sucha-,cyclic process. In such an operation, where thc brine is furthercooled and the crystallization of borax from the brine by contact with asludge of borax crystals is repeated, a part of the brine from thecrystallizer may be continuously circulated through a cooler and againsupplied to the same crystallizer and the remamder of the brine from thecrystallizer returned to the evaporators.

While the invention has been described particular-l in connection withapparatus for the crystal ization of borax from brines produced byconcentration of Searles Lake brine, it will be apparent that it isuseful in other crystallizations, such as in the crystallization ofother inorgnic salts or of other crystallizable constituents ormaterials of brines or liquors or solutions in which water or otherliquid may be the solvent.

It Will thus be seen that the lpresent invention provides an improvedapparatus for crystallizing dissolved crystallizable constituents fromliquors in which they are dissolved in which the liquor after beingbrought to supersaturation with the constituent it is desired tocrystallize is circulated in countercurrent flow with a sludge ofcrystals of the same constituent. The sludge of crystals in thecrystallizer is moving as well as'the supersaturated brine which isVflowed therethrough, the crystals of the sludge growing by er`stallization of the same constituent from t e supersaturated brine andbein removed from the operation as they reach t e desired size. At

' the same time, the crystals forming the sludge moving countercurrentto the su ersaturated brine are continuously supplie by crystallizationfrom the brine so` that the operation can be carried out in a continuousmanner. The large surface area of the cr stals in the sludge provides alarge crystal ization surface promoting rapid crystallization enablingthe operation to be carried out rapidly and in compact and eiicientapparatus. It vvill also be apparent that the present inventlon providesan improved apparatus for the direct crystallization of crystallizableconstituents Y fromv such liquors in large crystals, and in `acontinuous o eration, avoiding cumbersome crystallizing operation.

What I claim is:

1. An improved crystallizer comprising a vertically arranged shell, saidshell being relatively high with reference to its diameter, an agitatorarranged within the shell, means for supplying liquid to the shell andmeans for removing crystal sludge from the lower end of the shell, anenlarged chamber at the upper end of the shell and means for overflowinliquid therefrom, and means arrange within said chamber for propellingsettled and suspended material into said shell.

2. An improved crystallizer comprising a vertically arran ed shellrelatively high wlth reference to its iameter and a superimposed chambercommunicating with it at its upper end, an agitator shaft extendinthrough theV shell and the chamber, means or supplying liquid to andmeans for removing crystal sludge from the lower end of the shell, meansfor overflowing liquid from the upper end of the chamber, means in theshell connected with the said shaft for agitating liquid therein withoutgiving it any substantial component of vertical motion, and means in thechamber for givin liquid therein a down- Ward component o motion.

In testimony whereof I allix m si nature.

WILLIAM E. U KE.

